The “dead time” of a measurement instrument, such as an oscilloscope, is a time when data acquisition circuitry does not respond to a valid trigger event by displaying a waveform representing an electrical signal being monitored. In an analog oscilloscope, for example, dead time occurs during beam retrace time on a cathode ray tube. In a digital oscilloscope dead time often occurs when the instrument is busy with a number of tasks, i.e., not able to process a trigger event when a previously acquired waveform is being read from an acquisition memory or when the instrument is busy drawing the acquired data to make an image of the waveform for display.
Electronic circuits occasionally work in an unexpected manner. The incorrect operation may be rare, perhaps occurring once in thousands of correct cycles of operation. Fast circuits that cycle quickly often operate at rates much faster than a standard digital oscilloscope can display the corresponding waveforms. The typical digital oscilloscope ignores most of the trigger events because it is busy processing and drawing waveforms between acquisitions. The waveforms that show the incorrect operation of the circuit may be missed, i.e., may occur during the dead time between acquisitions. An oscilloscope user may have to wait a long time in order to view the incorrect operation. Even though incorrect circuit operation may not be visible, the oscilloscope user may not ever have confidence that the circuit is working properly since only a small fraction of the waveform data representing the input signal are drawn on the oscilloscope display. The basic digital oscilloscope has an architecture in which data is received in an acquisition memory, and then acquisition is halted by a trigger event while the acquisition memory is read and the waveform drawing is performed.
Co-pending U.S. patent application Ser. No. 11/388,428, filed by Steven Sullivan et al on Mar. 24, 2006 entitled “No Dead Time Data Acquisition”, is one attempt to enable the acquisition of all trigger events. A measurement instrument receives a digitized signal representing an electrical signal being monitored and generates from the digitized signal a trigger signal using a fast digital trigger circuit, the trigger signal including all trigger events within the digitized signal. The digitized signal is compressed as desired and delayed by a first-in, first-out (FIFO) buffer for a period of time to assure a predetermined amount of data prior to a first trigger event in the trigger signal. The delayed digitized signal is delivered to fast rasterizers or drawing engines upon the occurrence of the first trigger event to generate a waveform image. The waveform image is then provided to a display buffer for combination with prior waveform images and/or other graphic inputs from other drawing engines. The contents of the display buffer are provided to a display at a display update rate to show a composite of all waveform images representing the electrical signal. Two or more drawing engines may be used for each input channel of the measurement instrument to produce two or more waveform images, each waveform image having one of the trigger events of the trigger signal at a specified trigger position within a display window. The waveform images are combined to form a composite waveform image containing all the trigger events for combination with the previous waveform images in the display buffer and/or with graphics from other drawing engines. For certain trigger positions within the display window, an indicator is provided to show that a trigger event may have been missed. Also when there are no trigger events, a graphic of the signal content may still be provided for the display.
In the above-mentioned '428 U.S. patent application it is assumed that every display frame is drawn so as to include all trigger events over a sequence of display frames. As discussed in co-pending U.S. patent application Ser. No. 12/056,159, filed Mar. 26, 2008 by Kenneth P. Dobyns, et al entitled “Improved Holdoff Algorithm For No Dead Time Acquisition” [Attorney Docket Number 8257-US], to assure that all data related to the trigger events is displayed, an improved hold-off algorithm is disclosed. Unfortunately in order to assure that all relevant data is displayed, the display frames are drawn in an overlapping fashion, with duplicated data being drawn on the display screen multiple times. This approach to drawing waveforms may result in a confusing display for the instrument user.
Referring now to FIG. 1, a single glitch in the input signal is drawn in two places on the display screen with respect to a trigger point. In response to a first trigger event a first waveform is captured and drawn in a first display frame A. The second trigger event that has the glitch, since it occurs within the first display frame, does not form the basis for a new display frame and is therefore ignored. In response to a third trigger event which occurs outside the first display frame A, a waveform is captured and drawn in a second display frame B which coincidentally also includes the second trigger event. When the first and second display frames are combined for display, the glitch appears drawn in two places as shown. Although this is correct in that the glitch does occur in both places with respect to trigger events one and three, and by drawing a display frame for each trigger event the glitch may be drawn on all three pulses to be the most accurate, this is not the behavior expected by the user and may cause confusion.
Another similar example is shown in FIG. 2 where a burst of three pulses is drawn so it appears that there are five pulses on the instrument display screen. This is correct, assuming edge triggering, but it is still confusing for the user.